This invention relates to automated apparatus and methods for calibrating wire bonding apparatus and, more particularly, to wedge bonding systems which utilize pattern recognition for calibrating the stored coordinates of the center of rotation of the pedestal which carries a chip-containing package to be bonded.
In the prior art, integrated circuit chips are known to be electrically coupled to lead frames by wire bonding apparatus which bonds wires from a plurality of chip pads to corresponding beams or leads on a lead frame with which the chip is packaged.
As the circuit density of integrated circuit chips has increased, the accurate positioning of a wire bonding head utilized in automatic wire bonding apparatus to form wire bonds between chip pads and leads has been more difficult and has required more complex and sophisticated design. U.S. Pat. No. 4,441,205, Berkin et al, assigned to be assignee of the present invention, discloses a pattern recognition system which provides a successful solution to the problem of locating the chip so that the wire bonding head can the accurately positioned with respect to the chip. Further, co-pending U.S. application Ser. No. 735,839 Raghavan et al, entitled "Video Bond Lead Locator", and assigned to the same assignee, provides a solution to the problem of precisely determining the location of the leads of an integrated circuit chip lead frame so that the wires may be bonded to those leads without error which can otherwise result from failing to distinguish leads from specular reflection originating in the lead frame package.
However, in wire bonding apparatus of the type referred to as a wedge bonding system there has existed a persistent problem which has only been dealt with manually in the past, causing significant operating down time and reduction of throughput, and for which there exists a need for an automatic solution. In such systems, all bonding is done along a defined axis, e.g., along the Y axis of the field of movement of the bonding head. In the wedge bonding operation the thin wire, in the range of 0.7 to 3.0 mils and typically about 1.25 mils, must not be twisted or rotated. For this reason, the bonding head is constrained to move on a straight line while the work piece is rotated relative to it. Thus, for each successive bonding step the pedestal holding the work piece, or package containing the chip, is rotated by a pre-determined angle so that the chip pad and lead which are to be bonded lie on the Y axis along which the bonding head is allowed to move. Since the apparatus must tell the bonding head exactly where each pad is located, the apparatus must have very precise information about the location of the center of rotation of the pedestal. In a commercial bonder of the assignee, the pedestal rotation is referred to as theta motion. Note that in apparatus of this sort, as taught in the above-referenced Berkin et al patent, it is possible to determine quite accurately the location of a chip with respect to a reference point, such that if the actual center of rotation is also known, then all necessary information is on hand in order to determine the location of the bonding pad and lead pad after theta motion of a pre-determined amount.
Generally, in prior art operation of such wedge bonding apparatus, the real center of rotation is determined at the beginning of a run. It is stored in the memory of the apparatus control system, and is used thereafter until there is operator detection of slippage in locating the proper bonding positions. A problem which arises to cause such slippage is that the real center of rotation is displaced with respect to the system XY reference and bonding head due to thermal drift and other effects in the system. These systems contain substantial amounts of metal in order to absorb the rapid motions that are involved, as well as substantial linkages for driving the bonding head and the pedestal. It has been observed that following start-up of such an apparatus, the temperature rises and in fact the temperature of much of the metal in the apparatus changes substantially, causing the aforesaid thermal drift of the pedestal relative to the bonding head, such that the stored coordinates of the center of rotation come into error.
We have determined that when the actual center of rotation of the pedestal, or of the theta motion, differs from the calculated center, a theta rotation of 180.degree. can double the error in the placement of a bond. Consequently, this thermal drift requires possible frequent re-calibration of the bonder to update the software record of the actual center of rotation position during periods of temperature instability. For example, in practice we have found that the center of rotation can drift approximately one mil over a space of about 15 minutes. When non-automatic operator intervention is required to re-calibrate the center of rotation, this involves effective system down time of two-three minutes as frequently as every fifteen minutes, resulting in a significant decrease in throughput. While re-calibration may be required at different rates for different operations, the potential need for operator intervention requires the attendance of an operator for a substantial percentage of system operating time. Clearly, it is preferred to provide automatic re-calibration so as to continuously minimize and control drift, avoid system down time, increase throughput and minimize operator time.
In our approach to a solution to the above problem, we have chosen to utilize the signature generation and finding techniques as set forth in the above-referenced Berkin et al U.S. Pat. No. 4,441,205, which is incorporated herein by reference. The referenced Berkin et al patent discloses a preferred technique for generating a signature, and as used herein a signature may be either a single signature or an orthogonal signature, e.g, X and Y signatures comprising arrays of digital data corresponding to a pre-determined field associated with a given point on the target chip. As is also set forth in the above-referenced Berkin et al patent, a signature is "found" by scanning an area and generating a series of signatures, comparing each generated signature with the referenced signature and determining the minimum comparison difference. It is to be noted that while the signature generating and finding techniques set forth in the Berkin et al patent are preferred, other equivalent forms of signature generation and finding using pattern recognition can be utilized in the apparatus and method of this invention. This invention may use other pattern recognition or image processing systems, and the term signature includes templates and transforms of stored data.